Apoptosis or programmed cell death is a genetically and biochemically regulated mechanism that plays an important role in development and homeostasis in invertebrates as well as vertebrates.
Aberrancies in apoptosis that lead to premature cell death have been linked to a variety of developmental disorders. Deficiencies in apoptosis that result in the lack of cell death have been linked to cancer and chronic viral infections.
Caspases are cysteine-containing aspartate specific proteases that play a key role in effecting apoptosis. Once activated from their inactive zymogen form by proteolytic processing, caspases digest vital cell proteins from within the cell. Since caspases are such strong proteases, tight control of this family of proteins is necessary to prevent premature cell death. In addition to proteolytic processing, caspases are also regulated by a family of molecules known as Inhibitors of Apoptosis Proteins (IAP). IAPB are naturally occurring intracellular proteins that suppress caspase-dependent apoptosis. SMAC, an intracellular protein also known as DIABLO, functions to modulate the activity of IAPB. In normal healthy cells, SMAC and IAPB function together to maintain healthy cells. However, in certain disease states, e.g., cancers and other proliferative disorders, the activities of IAPB are not adequately modulated and therefore, prevent apoptosis and cause or exacerbate abnormal proliferation and survival.
IAP antagonists, also known as SMAC mimetics, are synthetic molecules that mimic the structure and IAP modulating activity of the four N-terminal amino acids of SMAC (AVPI). When administered to a subject suffering proliferative disorders, the compounds antagonize IAP activities causing an increase in apoptosis among abnormally proliferating cells.
IAPB are found in all organisms ranging from Drosophila to human and are known to be overexpressed in many human cancers. IAPB comprise one to three Baculovirus IAP repeat (BIR) domains. The BIR domain is a zinc binding domain of about 70 residues comprising 4 alpha-helices and 3 beta strands, with cysteine and histidine residues that coordinate the zinc ion. The BIR 2 and 3 domains contain a conserved inhibitor of apoptosis binding motif (IBM) capable of binding caspases—and inhibiting their proteolytic activity.
As an example, human X-chromosome linked IAP (XIAP) inhibits the executioner caspases-3, and -7 as well as the Apaf-1-cytochrome C mediated activation of the initiator caspase-9. Caspases-3 and -7 are inhibited by the BIR2 domain of XIAP, while the BIR3 domain of XIAP is responsible for the inhibition of caspase-9 activation. XIAP is expressed ubiquitously in most adult and fetal tissues. Overexpression of XIAP in tumor cells has been demonstrated to confer protection of the tumor cells against a variety of pro-apoptotic stimuli and promotes resistance to chemotherapy. Consistent with this, a strong correlation between XIAP protein levels and survival has been demonstrated for patients with acute myelogenous leukemia.
Other BIR2-3 containing IAP family members, while capable of binding caspases, do not directly inhibit their proteolytic activity. Rather they inhibit apoptosis by affecting signaling activities of key proteins in cell survival pathways. Like XIAP, these IAPB possess a carboxyl-terminal RING finger domain capable of conjugating ubiquitin to specific protein substrates. As an example, cellular IAPB 1 and 2 (cIAP1/2), ubiquitinate RIPK, a signaling intermediate of tumor necrosis death receptor (TNF-DR) activation. Ubiquitinated RIPK is unable to activate caspase-8 in the context of DR activation by TNF family DR ligands. On the contrary, the long ubiquitin chains attached to RIPK provide a scaffold by which cell components of the NFκB cell survival signaling cascade can attach and become activated.
In normal cells undergoing apoptosis, the IAP-mediated inhibition is removed by the mitochondrial protein SMAC (second mitochondrial activator of caspases; also known as DIABLO). SMAC is synthesized as a precursor molecule of 239 amino acids; the N-terminal 55 residues serving as the mitochondria targeting sequence that is removed after import. The mature form of SMAC resides in the inter-membrane space of mitochondria. At the time of apoptosis induction, SMAC is released from mitochondria into the cytosol where, together with cytochrome c, it binds to XIAP, and eliminates its inhibitory effect on caspases. SMAC also binds cIAP1/2 and inhibits their ability to ubiquitinate RIPK. SMAC interacts with essentially all IAPB that have been examined to date and thus appears to be a master regulator of apoptosis in mammals.
Down-regulation of XIAP expression by antisense oligonucleotides has been shown to sensitize tumor cells to death induced by a wide range of pro-apoptotic agents, both in vitro and in vivo. SMAC/DIABLO-derived peptides have also been demonstrated to sensitize a number of different tumor induced select cell lines to undergo apoptosis as single agents, while other cell lines require an additional stimulus such as DR agonists or co-treatment with pro-apoptotic drugs. Because IAP inhibition appears to be a viable mechanism for promoting apoptosis and treating diseases and conditions that are sensitive to apoptosis, there is a continuing need to develop compounds that can inhibit IAP.